The invention relates to solar collectors, and more specifically to an "open flow" type collector of economic materials, particularly concrete.
Solar energy collectors made of concrete have been known at least since 1904, when H. E. Willsie published his work "Experiments in the Development of Power From the Sun's Heat", Engineering News, N.Y., Vol. 61, No. 19, 1909. H. E. Willsie's collector can be described as an "open-flow", "gravity flow", "sheet flow" or "trickle" type solar energy collector. Such an open-flow energy collector made of concrete has many generic advantages such as durability, low-cost, low energy investment (and therefore short energy payback), and great flexibility in shape, size and other structural features.
However, and in spite of a large body of recent work on open-flow or sheet-flow energy collectors, there remain a number of unsolved problems pertaining to concrete open-flow solar collectors. My invention solves these problems, particularly the following:
1. High heat losses.
This category involves three different problem areas:
(a) Low insulation value of concrete, with the added problem of a further decrease in the insulation value of concrete resulting from its water permeability.
(b) High heat losses due to heat transfer from the photothermal conversion surface to the glazing, occurring when water vaporizes at that surface and condenses on the inside of the glazing.
(c) Radiation heat losses from the photothermal conversion surface that occur when the water film is not thick enough to filter the thermal infra-red reradiation.
2. Incomplete wetting of the photothermal conversion surface.
This is a simple mechanical problem, but failure to find a satisfactory solution drastically cuts the efficiency of the solar energy collector in direct proportion to the unwetted surface.
3. Air bubbles.
In the case of sheet-flow, where a water film is flowing directly between a photothermal conversion surface and a glazing, air bubbles tend to form on the glazing and, through a combination of electrostatic and hydrostatic forces, acquire spatial stability. These air bubbles increase reflection of impinging solar energy and disrupt the continuity of the heat transfer fluid sheet flow over the photothermal conversion surface, with adverse effects on collection efficiency.
4. Deposits.
The photothermal conversion surface in typical open-flow collectors is exposed daily to the drying of the water film, and with most types of water, tap or well, this repeated drying, sometimes under high insolation, will cause deposits of salts and minerals that were in suspension or solution in the water. Such deposits can quickly cut down on the absorption characteristics of the photothermal conversion surface.
5. Cost of construction, transportation and erection.
Thin concrete edges are brittle and should be avoided, and the concrete collector should be designed to simplify handling, transportation and installation.
The patents and publications in the following brief survey represent the most pertinent prior art pertaining to open-flow and concrete open-flow or sheet-flow collectors of which I am aware.
H. E. Willsie (U.S. Pat. No. 1,101,001) does not provide explicit means to insure wetting of the whole photothermal conversion surface, nor does he contain the water sheet to prevent heat loss through evaporation. Yu's (U.S. Pat. No. 3,943,911) disclosure is similar in some broad respects to my invention, but does not disclose any means to insure wetting of the whole conversion surface, any means to decrease reradiation from the conversion surface, any means to increase absorption, nor any means to prevent deposits buildup. The same can be said of Meier et al. (U.S. Pat. No. 4,056,092) invention, or of the collectors described by P. R. Payne and D. W. Doyle, "The Fossil Fuel Cost of Solar Heating" Society of Automotive Engineers, 1978. In addition, the latter collectors do not prevent evaporation. Isaacson (U.S. Pat. No. 4,153,037) discloses a grooved conversion surface on a plastic or glass-based collector, but without concern for the shape or size of the grooves, and further without means of insuring even flow over all the grooves, or containment of the fluid sheet. Robinson, Jr. et al. (U.S. Pat. No. 4,074,705) discloses a header to evenly spread the heat transfer fluid, but of a cumbersome design. Scheffee (U.S. Pat. No. 4,170,984) is concerned with "Flooded Flow" but uses increased viscosity of the heating fluid to achieve it. Viestur et al. (U.S. Pat. No. 4,193,543) discloses an arrangement to regulate the flow of the heating fluid somewhat reminiscent of Willsie's, but that system does not provide for an even flow over all the conversion surface, and the system shares the same shortcomings with Yu and Meyer.
It is among the objects of the invention to improve on prior open-flow, flat plate type solar collectors by providing efficient solutions in the above problem areas, resulting in an open-flow or sheet-flow collector of greatly improved efficiency and cost effectiveness. Other objects, advantages and features will be apparent from the following description of a preferred embodiment, considered along with the accompanying drawings.